Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
756 PM EDT Fri Jul 09 2021
Valid 12Z Tue Jul 13 2021 - 12Z Sat Jul 17 2021
...Overview...
Not a lot has changed over the past day with respect to the
general themes of the large scale pattern as well as the
uncertainties within this pattern. Guidance continues to show a
fairly blocky regime for most of next week from the Bering
Sea/Aleutians into Mainland Alaska and the Northeast Pacific. At
the start of the period early Tuesday an upper high should be
centered near the Bering Strait with ridging that extends into
portions of the mainland while an upper low should be east of
Kodiak Island. Some weak energy should exist over/south of the
southern Bering Sea. With time the upper ridge/high will likely
weaken and drop southward while the upper low gradually tracks
closer to the southern Panhandle. The upstream energy should
support a wave that likely weakens as it tracks eastward from the
Aleutians. The combined evolutions of the upper ridge and low as
well as the energy to the south/west continue to vary in the
guidance. Meanwhile at higher latitudes, there is decent
agreement that an initial Arctic trough will head into the
Canadian Archipelago while a surface front pushes into the
northern/eastern mainland through midweek. Thereafter a rapid
increase of spread for details of upstream flow--which leads to a
wide variety of possibilities for how the mean front may oscillate
from day to day--dramatically lowers confidence in forecast
specifics. Finally, guidance continues to signal the development
of a system reaching near the western Aleutians around the end of
the week.
...Guidance/Predictability Evaluation...
Today's blend started with a 12Z operational model composite
(though splitting ECMWF input between the 12Z and 00Z runs) from
day 4 Tuesday into day 5 Wednesday. After that time the forecast
rapidly phased out the 12Z GFS due to its low-probability
evolution of the upper low east of Kodiak Island wobbling
southwestward and merging with the energy arriving from the
Aleutians. Sure enough, the new 18Z GFS abandoned that idea and
returned closer to consensus. There is a fair amount of spread
for the weak Aleutians wave with 12Z ECMWF/UKMET runs noticeably
east/southeast versus continuity and consensus. The new 12Z ECMWF
mean trended faster as well though. Especially to yield a
conservative forecast at higher latitudes, the blend quickly
increased 12Z GEFS/00Z ECens weight to 70 percent by days 7-8
Friday-Saturday. This solution shows relatively flat mean flow
aloft and a persistent front over the northern mainland through
the end of the period but with an ill-defined pattern to the north
after early Thursday. Adding in trends from the new 12Z ECMWF
mean and 18Z GFS, there seems to be increasing potential that an
Arctic wave will pull the front back to the north after midweek
and then bring the mean front back to the south in its wake.
Future forecasts will refine this evolution as a better consensus
develops. Thus far the ensemble means have been fairly agreeable
and consistent for the system expected to reach near the western
Aleutians by the end of the week. Operational runs have displayed
typical variance, with latest trends appearing to lean away from
some solutions that had been faster than the ensemble means. This
provides support for a slightly enhanced version of the means in
principle.
...Weather/Threats Highlights...
Mostly light to moderate precipitation initially near the southern
coast/Panhandle should expand northward into midweek with a brief
period of greater focus and potentially somewhat heavier intensity
possible over and near Southcentral. Most of the southern
mainland should see a drier trend late week into the weekend while
parts of the Panhandle could see continued periods of light
activity ahead of the Gulf/Northeast Pacific low. Expect the
front reaching the northern mainland to focus some precipitation
of varying intensity. After midweek there is considerable
uncertainty regarding the position of this front and associated
precip. To the southwest, a weak Aleutians wave may produce some
light rainfall early in the period. The system forecast to reach
the western Aleutians around the end of the week may support a
leading area of organized rain and stronger winds.
The Panhandle and portions of the southern mainland are likely to
see below normal temperatures for most of the period, and
especially through midweek or so when clouds and precipitation
should be a greater factor. A cooling trend behind the front
dropping into the northern mainland will yield below normal
temperatures over the North Slope by Wednesday-Thursday. During
the late week/weekend time frame temperatures over this region
will depend on frontal position that has low confidence for now.
Interior areas should see mostly above normal highs Tuesday
followed by a trend toward modestly above/below normal readings,
with some dependence on exactly how far south the northern
mainland front reaches. Interior and southwestern areas will tend
to see above normal low temperatures on most days.
Rausch
No significant hazards are expected over Alaska during this
forecast period.
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html